1. Field of the Invention
The present invention relates generally to packaging integrated circuit devices and in particular to providing electrical discharge properties to integrated circuit device packaging. Still more particularly, the present invention relates to forming a metal ring around an integrated circuit from a portion of a lead frame for the purpose of conducting electrostatic energy away from the integrated circuit.
2. Description of the Prior Art
In conventional integrated circuits, electrostatic discharge (ESD) events typically enter the circuitry through the pad ring, which dissipates the charge before reaching the core. Some recently developed integrated circuits, however, must necessarily expose the core of the circuitry to ESD events. Contemporary fingerprint sensors, for example, often include a two-dimensional array of sensing electrodes proximate to a sensing surface on which the finger is placed, with ridges and valleys on the finger skin detected by capacitance variations caused by the varying distance between the skin surface and the sensor electrodes. The need for contact with the finger in order to detect fingerprint features necessitates exposure of the integrated circuit to electrostatic discharge events resulting from a finger touching the sensing surface.
The electrostatic charge which may be carried by a human body often falls within the range of several kilovolts or more. Typical electrostatic discharge protection circuits have proven somewhat ineffective in safely dissipating such charges, which may provide sufficient energy to break through the upper dielectric/passivation layer.
Additionally, integrated circuits which cannot be completely encapsulated—except for conductive leads to the circuit—during packaging (e.g., fingerprint sensors, optical sensors, and other circuit requiring that a portion of the integrated circuit remain exposed) are typically mounted utilizing “Chip On Board” technology. The integrated circuits are mounted on a printed circuit board in an unencapsulated form, connected to the printed circuit board through the bond wires, then protected utilizing liquid encapsulation or silicon gel. Such mounting, which involves the use of gold plating of the substrate, is much more expensive than conventional surface mounting of integrated circuits on a stamped lead frame. However, viable surface mounting of fingerprint sensors on lead frames has not yet been achieved.
It would be desirable, therefore, to provide a technique for surface mounting of fingerprint sensors on lead frames while providing adequate electrostatic discharge protection to the packaged integrated circuit.